In the paper field and, in particular, papers for household and sanitary use, many multi-ply products are known wherein several previously embossed paper plies are joined to form a laminate. As to this kind of product, the attempt is made to combine softness, in order to create a pleasant touch, and absorption with good properties of mechanical strength. As a rule, these products are made by joining and bonding the previously embossed paper plies to one another at discrete points.
In that procedure, a glue-depositing device deposits glue on a first and previously embossed ply at the tip of a projection made during embossing. Then, this first ply is made to touch another ply also previously embossed, so as to achieve bonding between the two plies at their contact points.
Depending on the relative positions of the embossings at the time the plies are being bonded, such techniques basically allow for two kinds of assembly. For example, U.S. Pat. No. 3,414,459 describes an assembly whereby the embossing tips are mutually opposite, whereas U.S. Pat. No. 3,867,225 describes an assembly whereby the embossing tips of one ply will nest between the embossing tips of the other ply.
Since embossing has been used, the manufacturing method for such products restrict the diversity of patterns that can be imprinted into the paper because such systems can only use engravings of rounded patterns or patterns lacking sharp angles in order to restrict the losses in mechanical strength during the embossing. What is of concern is to avert breaking the fibers of the plies.
Moreover, it is desirable to minimize the amount of glue used, especially to avoid rigidity.
Accordingly, it has previously been suggested to mark and assemble two plies without using glue. Illustratively, such products are described in U.S. Pat. No. 4,281,243. Therein two bonded plies are linked solely at their edges. This feature does not allow for the fact that it would be impossible to preserve the plies' mechanical strength over their entire surface from the applied and required high pressures, for instance by knurling, to achieve adequate linkage.
U.S. Pat. No. 3,672,549 suggests a procedure for bonding two previously embossed plies at discrete points. As shown by FIG. 2 of the patent showing apparatus with which to carry out the procedure, it is noted that in order to achieve the discrete bonding of the two plies, the apparatus employs three cylinders 27, 28 and 30. Two of these cylinders, i.e., 27 and 30, are rubber-coated and have engraved surfaces. The third, i.e., 28, however, is made of smooth steel. The cylinder 27 cooperates with a glue-depositing device and carries out simultaneously glue deposition and marking of ply 10. The two plies 9 and 10 are bonded when they pass between the cylinders 28 and 30. In particular, because the system makes use of the smooth cylinder 28, the system runs the risk of the ply slipping relative to the cylinder surface. Such slippage can result in irregular final patterns or in breaks in the paper.
The system incurs another drawback in that the glue is deposited on an engraved surface and then is transferred to the paper. In order to prevent the glue on the rubber surface from drying and from soiling the rubber surface, a large amount of glue is required. Therefore, it is impossible to make multi-ply and low-weight products in which the amount of dragged-away glue would have been much reduced.
Another drawback incurred by this ply marking and bonding system arises from the relative positions of the cylinders 27, 28 and 30. The final product is never. symmetrically marked on the two outer sides of the laminate formed from the plies 9 and 10 since one of the two markings is located inside the formed laminate at the time the two plies are bonded together.